The field of the invention is the control of anode current in an x-ray tube and, particularly, the precise control of anode current in an x-ray tube of the type used in CT scanners.
As shown in FIG. 1, an x-ray tube 10 includes a thermionic filament 11 and an anode 12 which are contained in an evacuated envelope 13. An ac current IF of 2-6.5 amps is applied to the filament 11 causing it to heat up and emit electrons. A high dc voltage of from 50 to 150 kilovolts is applied between the filament 11 and the anode 12 to accelerate the emitted electrons and cause them to strike the target material on the anode 12 at high velocity. X-ray energy indicated by dashed line 14 is emitted as a result.
The amount of x-ray energy which is produced is determined by the high voltage level and the amount of tube current I.sub.T which flows between the filament 11 and the anode 12. The high voltage is set to a selected value and the high voltage power supplies 15 and 16 maintain that value during the entire scan. The tube current I.sub.T is controlled by controlling the amount of filament current I.sub.F, and this in turn is controlled by the ac voltage produced at the secondary winding of a filament transformer 17. The relationship between tube current I.sub.T and applied filament current is nonlinear and is typically exponential.
In a CT scanner, it is common practice to change the filament current between scans in order to change the level of x-ray production. Consequently, the filament current control circuit must be capable of rapidly bringing the filament current to a level which results in the desired x-ray tube current I.sub.T before each scan is begun.
In CT scanning, a high degree of precision is required in the amount of x-rays produced since the attenuation data is sequentially obtained during the entire scan procedure and the method employed to reconstruct an image from this acquired data presumes that the x-ray energy remains constant during the entire scan. This requires that tube current I.sub.T be very precisely controlled.
Referring still to FIG. 1, these requirements are met by filament current control systems which operate in an open loop mode during the preheating of the filament and a closed loop mode when x-rays are produced and tube current I.sub.T is to be precisely controlled. During the open loop mode of operation, a preheat current command is applied to the input of a digital-to-analog (D/A) converter 20 by a digital control system (not shown). The resulting analog preheat current command is amplified by amplifier 21 which also limits the magnitude of the command to a safe level, and the resulting signal is input to a filament driver 22. The filament driver 22 produces an ac output voltage that is applied to the primary of the filament transformer 17 and which produces the commanded filament current I.sub.F. A filament current feedback signal produced by a current sensor attached to the primary or secondary of the filament transformer 17 is fed back through line 23 to force the filament current I.sub.F to the desired level by closed loop control action.
A short time interval later the high voltage is turned on to produce x-rays, and the current control system is switched to its closed loop mode of operation. This is accomplished by closing an analog switch 25 with a command signal from the digital control system through line 26. This applies a feedback signal to a summing point 27 at the input of amplifier 21 that adds to the preheat current command and adjusts the filament current I.sub.F to a point which produces the desired x-ray tube current I.sub.T.
The tube current I.sub.T is measured by a resistor 30 which is connected in series with the high voltage power supplies 15 and 16 and which is connected across the inputs of an operational amplifier 31. In a high performance system, this tube current feedback signal is summed with a tube current command signal at an error amplifier 32 and the difference, or error, signal is applied to the input of a variable gain amplifier 33. The tube current command is typically issued in digital form by the digital control system and is converted to an analog command signal by D/A converter 34. The tube current command signal is the value which determines the amount of x-rays that are to be produced during the scan at the selected high voltage level. The resulting feedback signal produced by amplifier 33 forces the actual tube current I.sub.T to equal the tube current command by controlling the filament current I.sub.F through feedback control action at the summing point 27.
To maintain steady state accuracy and the desired transient response, the overall gain and phase of the tube current feedback loop should be maintained constant over the entire operating range, which may be from under 10 milliamperes to over 1,000 milliamperes in a CT x-ray tube. However, it is well known that the transfer function of the x-ray tube, defined as the incremental change in tube current I.sub.T caused by an incremental change in filament current I.sub.F, is dependent on the level of the tube current I.sub.T. As a result, to achieve high performance throughout its operating range prior current control systems include the variable gain amplifier 33 in the tube current feedback loop to compensate for the variability of the tube transfer function to obtain roughly constant loop gain. That is, each time the tube current command is changed, a gain command is also applied to the variable gain amplifier 33 through line 35 to adjust the loop gain and to thereby accommodate the different x-ray tube transfer function brought about by the different tube current I.sub.T. If the loop gain is not maintained at a relatively constant level, the control system is inaccurate and responds poorly at low tube current levels and may be unstable at high tube current levels.